CS 372 – introduction to computer networks*
Lecture 3: Wednesday June 23
Announcements:
 Assignment 1 is posted online and is due next
Tuesday
 Quiz on next Tuesday
 Lab 1 is posted and is due next Monday
 No late lab and assignment will be accepted!
* Based in part on slides by Bechir Hamdaoui, Paul D. Paulson, and Dina Katabi.
Acknowledgement: slides drawn heavily from Kurose & Ross
Chapter 1, slide:
1
The network core: Packet switching
 Data transmitted in small, independent
pieces
Source divides outgoing messages into packets
 Destination recovers original data

 Each packet travels independently
 Includes enough information for delivery
 May follow different paths
 Can be retransmitted if lost
Chapter 1, slide:
2
The network core:
Functions of packet-switching networks
 Packet construction
 encode/package data at source
 Packet transmission
 send packet from source to destination
 Packet interpretation
unpack/decode data from packet at destination
 acknowledge receipt

Chapter 1, slide:
3
The network core: other functions
 Route discovery
 Traffic/congestion control
 Retransmitting lost packets
 Determining type of data
 messages
 service requests/responses
 files
 audio/video
 etc.
 etc.
Chapter 1, slide:
4
Packet switching: Reordering and
different path
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Node 6
Node 7
Host E
Node 4
Chapter 1, slide:
5
Chapter 1: roadmap
1 What is the Internet?
2 Network edge
3 Network core
4 Network access and physical media
5 Internet structure and ISPs
6 Protocol layers, service models
7 Delay & loss in packet-switched networks
Chapter 1, slide:
6
Access networks and physical media
Q: How to connect end
systems to edge router?
 residential access nets
 institutional access
networks (school, company)
 mobile/wireless access
networks
Chapter 1, slide:
7
Physical Media
 why is it needed?
 to propagate bits between sender/receiver pairs
 what is it?
 a physical link that lies between sender & receiver
 two types of media:
 guided media: signals propagate in solid media

unguided media: signals propagate freely, e.g.,
wireless radio
Chapter 1, slide:
8
Residential access: point to point access
 Dialup via modem
regular twisted-pair copper
phone lines
 up to 56Kbps direct access to
router (often less)
 rate depends on thickness
and distance
 may pick up interference
(“noise”)
 can’t surf and phone at same
time: can’t be “always on”

Chapter 1, slide:
9
Residential access: point to point access
 ADSL: asymmetric digital subscriber line
regular phone lines
 transmission rates depend on length
 point-to-point medium (dedicated)
 up to 1 Mbps upstream

(today typically < 256 kbps)

up to 8 Mbps downstream
(today typically < 1 Mbps)

FDM: 50 kHz - 1 MHz for downstream
4 kHz - 50 kHz for upstream
0 kHz - 4 kHz for ordinary telephone
Chapter 1, slide: 10
Guided Media: coaxial cable
 two concentric copper conductors
 baseband:
 single channel on cable
 legacy Ethernet
 broadband:
 multiple channels on cable
 hybrid fiber-coax cable (HFC)
 Cable TV
 rate depends on thickness and distance
 less interference than twisted pair
Chapter 1, slide: 11
Residential access: cable modems
 HFC: hybrid fiber coax
asymmetric
 up to 30Mbps downstream
 up to 2 Mbps upstream

 network of cable and fiber attaches homes to
ISP router
 Shared medium
 deployment: available via cable TV companies
Chapter 1, slide: 12
Cable Network Architecture: Overview
Typically 500 to 5,000 homes
cable headend
cable distribution
network (simplified)
home
Chapter 1, slide: 13
Cable Network Architecture: Overview
server(s)
cable headend
cable distribution
network
home
Chapter 1, slide: 14
Cable Network Architecture: Overview
cable headend
cable distribution
network (simplified)
home
Chapter 1, slide: 15
Cable Network Architecture: Overview
FDM:
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1
2
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4
5
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7
8
9
Channels
cable headend
cable distribution
network
home
Chapter 1, slide: 16
Company access: local area networks
 local area networks (LAN), more in chapter 5
 connect end system to edge router
 E.g., universities, companies
Example:
 Ethernet:


shared or dedicated link
connects end system to router
10 Mbs, 100Mbps,
Gigabit Ethernet
Chapter 1, slide: 17
Wireless access networks
 wireless access network
 connects end system to router
 via base station or “access point”
Examples:
 wireless LANs:

router
base
station
802.11b/g (WiFi): 11 or 54 Mbps
 wider-area wireless access
 provided by telcomm operator
 3G ~ 384 kbps
 GPRS in Europe/US
mobile
hosts
Chapter 1, slide: 18
Chapter 1: roadmap
1 What is the Internet?
2 Network edge
3 Network core
4 Network access and physical media
5 Internet structure and ISPs
6 Protocol layers, service models
7 Delay & loss in packet-switched networks
Chapter 1, slide: 19
Internet structure: network of networks
 roughly hierarchical: tier 1, tier 2, and tier 3
 at center: “tier-1” ISPs
 e.g., MCI, Sprint, AT&T, Cable and Wireless,
 national/international coverage
Tier-1
providers
interconnect
(peer)
privately
Tier 1 ISP
Tier 1 ISP
NAP
Tier-1 providers
also interconnect
at public network
access points
(NAPs)
Tier 1 ISP
Chapter 1, slide: 20
Tier-1 ISP: e.g., Sprint
Sprint US backbone network
DS3 (45 Mbps)
OC3 (155 Mbps)
OC12 (622 Mbps)
OC48 (2.4 Gbps)
Seattle
Tacoma
Stockton
San Jose
Cheyenne
Kansas City
New York
Pennsauken
Relay
Wash. DC
Chicago
Roachdale
Anaheim
Atlanta
Fort Worth
Orlando
Chapter 1, slide: 21
Internet structure: network of networks
 “Tier-2” ISPs: smaller (often regional) ISPs
 Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
Tier-2 ISP is
customer of
tier-1 provider
Tier 1 ISP
Tier-2 ISP
NAP
Tier 1 ISP
Tier-2 ISPs
also peer
privately with
each other,
interconnect
at NAP
Tier-2 ISP
Tier-2 ISP
Chapter 1, slide: 22
Internet structure: network of networks
 “Tier-3” ISPs and local ISPs
 last hop (“access”) network (closest to end systems)
local
ISP
Local and tier3 ISPs are
customers of
higher tier
ISPs
connecting
them to rest
of Internet
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Chapter 1, slide: 23
Internet structure: network of networks
 a packet passes through many networks!
local
ISP
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Chapter 1, slide: 24
Chapter 1: roadmap
1 What is the Internet?
2 Network edge
3 Network core
4 Network access and physical media
5 Internet structure and ISPs
6 Protocol layers, service models
7 Delay & loss in packet-switched networks
Chapter 1, slide: 25
Protocol “Layers”
Networks are complex!
 many “pieces”:
 hosts
 routers
 links of various
media
 applications
 protocols
 hardware,
software
Question:
Is there any hope of
organizing structure of
network?
Or at least our discussion
of networks?
Chapter 1, slide: 26
Organization of air travel
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing
 a series of steps
Chapter 1, slide: 27
Layering of airline functionality
ticket (purchase)
ticket (complain)
ticket
baggage (check)
baggage (claim
baggage
gates (load)
gates (unload)
gate
runway (takeoff)
runway (land)
takeoff/landing
airplane routing
airplane routing
airplane routing
departure
airport
airplane routing
airplane routing
intermediate air-traffic
control centers
arrival
airport
Layers: each layer implements a service
 via its own internal-layer actions
 relying on services provided by layer below
Chapter 1, slide: 28
Why layering?
Dealing with complex systems:
 Easing assignment of tasks

identify relationship among pieces of complex
systems
 Easing maintenance, updating of system
change of implementation of layer’s service
transparent to rest of system
 e.g., change in gate procedure doesn’t affect
rest of system

Chapter 1, slide: 29
Internet protocol stack
 application: supporting network
applications

FTP, SMTP, HTTP
 transport: process-process data
transfer

TCP, UDP
 network: routing of datagrams from
source to destination

IP, routing protocols
 link: data transfer between
application
transport
network
link
physical
neighboring network elements

PPP, Ethernet
 physical: bits “on the wire”
Chapter 1, slide: 30
Encapsulation
source
message
segment
M
Ht
M
datagram Hn Ht
M
frame Hl Hn Ht
M
application
transport
network
link
physical
link
physical
switch
destination
M
Ht
M
Hn Ht
Hl Hn Ht
M
M
application
transport
network
link
physical
Hn Ht
Hl Hn Ht
M
M
network
link
physical
Hn Ht
M
router
Chapter 1, slide: 31
ISO/OSI Model: late 70’s
application
presentation
session
transport
application
transport
network
network
link
data link
physical
physical
7-layer ISO/OSI model
(OSI: open system interconnections)
5-layer Internet
Protocol Stack
Chapter 1, slide: 32
Chapter 1: roadmap
1 What is the Internet?
2 Network edge
3 Network core
4 Network access and physical media
5 Internet structure and ISPs
6 Protocol layers, service models
7 Delay & loss in packet-switched networks
Chapter 1, slide: 33
Network performance metrics
End-to-end delay (nodal delay) :
 Total time from initiating “send” (from
source) to completed “receive” (at
destination)
Throughput :
 Rate (bits/sec) at which bits are actually
being transferred between sender/receiver
instantaneous: rate at given point in time
 average: rate over longer period of time

Chapter 1, slide: 34
Sources of packet delay
 1. nodal processing:
 check bit errors
 determine output link
 2. queueing
 time waiting at output
link for transmission
 depends on congestion
level of router
A
B
nodal
processing
queueing
Chapter 1, slide: 35
Sources of packet delay
4. Propagation delay:
3. Transmission delay:
 R=link bandwidth (bps)
 d = length of physical link
 L=packet length (bits)
 s = propagation speed in
medium (~2x108 m/sec)
 propagation delay = d/s
 trans. delay = L/R
Note: s and R are very different quantities!
transmission
A
propagation
B
nodal
processing
queueing
Chapter 1, slide: 36
How do loss and delay occur?
packet being transmitted (delay)
A
B
packets queueing (delay)
packets get dropped (loss)
if no free buffers
Chapter 1, slide: 37
Packet loss
 queue (buffer) preceding link in buffer has
finite capacity
 packet arriving at a full queue is dropped (lost)
 lost packet may be retransmitted by previous
node, by source, or not at all
buffer
packet being transmitted
(waiting area)
A
B
packet arriving to
full buffer is lost
Chapter 1, slide: 38
Caravan analogy
100 km
ten-car
caravan
toll
booth
 Cars run at 100 km/hr (speed
of propagation)
 Booth takes 12 sec to service
a car (transmission time)
 car~bit; caravan ~ packet
 Q: How long until caravan is
lined up before 2nd toll
booth?
100 km
toll
booth
 Time to “push” entire
caravan through toll booth
= 12*10 = 120 sec = 2 mns
 Time for last car to
propagate from 1st to 2nd
toll both:
=100km/(100km/hr)= 1 hr
 A: 1 hr 2 minutes
Chapter 1, slide: 39
Caravan analogy (more)
100 km
ten-car
caravan
toll
booth
 Cars now “propagate” at
1000 km/hr
 Toll booth now takes 1
min to service a car
 Q: Will cars arrive to
2nd booth before all
cars serviced at 1st
booth?
100 km
toll
booth
 Yes! After 7 min, 1st car
at 2nd booth and 3 cars
still at 1st booth.
 1st bit of packet can
arrive at 2nd router
before packet is fully
transmitted at 1st router!
Chapter 1, slide: 40
Example
Packet length = L bits
Host A
trans. rate R = 1 Mbps
Host B
distance = 1 km, speed = 2x108m/s
Question:
 Which bit is being transmitted at the time the first bit
arrives at Host B for
Answer:
First bit arrives after
1/R + d/s = 1/106 + 103/(2x108) = 10-6 + 5x10-6 = 6x10-6 = 6 µsec
After 6 µsec
6 bits are already transmitted; so 7th bit is being transmitted
Chapter 1, slide: 41
Nodal delay
d nodal  d proc  d queue  d trans  d prop
 dproc = processing delay
 typically a few microsecs or less
 dqueue = queuing delay
 depends on congestion
 dtrans = transmission delay
 = L/R, significant for low-speed links
 dprop = propagation delay
 a few microsecs to hundreds of msecs
Chapter 1, slide: 42
Queueing delay (revisited)
Packet arrival rate
= a packets/sec
Packet length
= L bits
queue
Link bandwidth
= R bits/sec
 Every second: aL bits arrive to queue
 Every second: R bits leave the router
 Question: what happens if aL > R ?
 Answer: queue will fill up, and packets will get dropped!!
aL/R is called traffic intensity
Chapter 1, slide: 43
Queueing delay (revisited)
Packet arrival rate
= a packets/sec
queue
Packet length
= L bits
Link bandwidth
= R bits/sec
 La/R ~ 0: avg. queueing delay small
 La/R -> 1: delays become large
 La/R > 1: more “work” than can be
serviced, average delay infinite!
Chapter 1, slide: 44
Exercise 1
Transmission vs. propagation
L=100Bytes
Host A
Question:
trans. rate R = ?
Host B
distance = 2 km, speed = 2x108m/s
 At what rate (bandwidth) R would the propagation delay
equal the transmission delay?
Answer:
 Propagation delay = 2x103 (m)/2x108 (m/s) = 10-5 sec
 Transmission delay = 100x8 (bits)/R
 Prop. Delay = trans. Delay => R=105x100x8 = 80 Mbps
Chapter 1, slide: 45
Exercise 2
Voice over IP
L=48 Bytes
a=64Kbps
Host A
trans. rate R = 1Mbps
Host B
delay_prop = 2msec
 Host A

converts analog to digital at a=64Kbps
groups bits into L=48Byte packets
sends packet to Host B as soon it gathers a packet

As soon as it receives the whole pckt, it converts it to analog


 Host B
 Question:

How much time elapses from the 1st bit is created until the last
bit arrives at Host B?
Chapter 1, slide: 46
Exercise 2
Voice over IP
L=48 Bytes
a=64Kbps
Host A
trans. rate R = 1Mbps
delay_prop = 2msec
Host B
Answer:
 Time to gather 1st pkt: 48x8 (bits)/64x1000 (b/s) = 6 msec
 Time to push 1st pkt to link: 48x8 (bits)/1x106 (b/s) = 0.384 msec
 Time to propagate: 2 msec
 Total delay = 6 + 0.384 + 2 = 8.384 msec
Chapter 1, slide: 47
Introduction: Summary
Covered a “ton” of material!
 Internet overview
 what’s a protocol?
 network edge, core, access
network
 packet-switching versus
circuit-switching
 Internet/ISP structure
 performance: loss, delay
 layering and service
models
You now have:
 context, overview,
“feel” of networking
 more depth, detail to
follow!
Chapter 1, slide: 48